1. Field of the Invention
The invention relates to a projection exposure apparatus and method used in the lithography process of projecting a form, such as a mask or reticle, onto a wafer for the purpose of manufacturing devices, e.g., integrated circuits, imaging devices, liquid crystal displays, or thin film magnetic recording heads. More specifically, the present invention relates to a projection exposure apparatus and method of using soft x-rays called EUVL (Extreme Ultra Violet Lithography). The present invention also relates to an illumination optical system and method suitable for the projection exposure apparatus. Finally, the present invention relates to a method of manufacturing said devices by means of said apparatus.
2. Description of Related Art
A projection exposure apparatus for projecting a form, such as a mask or reticle, onto a wafer by means of soft x-rays (EUV radiation) is disclosed in documents such as Sweatt, William C. High-Efficiency Condenser Design for Illuminating a Ring Field. OSA Proceedings on Soft X-Ray Projection Lithography, Vol. 18 (1993) and U.S. Pat. No. 5,315,629.
In this projection exposure apparatus, a reflection type mask is used as a form, and the exposure is performed while the form is moved relative to the projection optics. In other words, a scanning exposure method is utilized for projecting the whole pattern of the form onto the shot area on the wafer (work). The exposure is performed while an exposure area, which is an image of a portion of the form, is being scanned onto the work.
In the projection exposure apparatus using soft x-rays according to the prior art described above, the exposure area should be limited in order that only the specific pattern on the mask is projected.
In performing the scanning exposure, in order to make the cumulative exposure equal at every point in the shot area, it is necessary to make the slit width along the scanning direction within the exposure area substantially equal at any point along the direction perpendicular to the scanning direction.
In performing the scanning exposure, there are two methods for limiting the exposure area: either a slit is placed near the reflection mask, or a slit is placed near the work. In the former method, the slit must be moved relative to the reflection mask while performing the exposure. In the latter method, on the other hand, the slit must be moved relative to the work while performing the exposure. Therefore, there must be a space between the slit and the reflection mask or the work, in order to avoid direct contact between the slit and the reflection mask or the work.
Instances where the distance between the slit and the reflection mask or between the slit and the work are determined are described with reference to FIG. 5. FIGS. 5A through 5C show the relation between the slit S and the light flux arriving at the reflection mask 9. FIG. 5A shows a case where the light flux arrives at the center 9A of the exposure area. FIGS. 5B and 5C show cases where the light flux arrives at the corners 9B and 9C of the exposure area. Since the light flux arriving at the center of the exposure area does not produce vignetting, as seen from FIG. 5A, the shape of the aperture of the projection optical system becomes a circle as shown in FIG. 5D. Because the light flux arriving at 9B and 9C, on the other hand, produce vignetting, as seen by the hatched portion in FIGS. 5B and 5C, the shape of the aperture of the projection optical system shows a portion of the circle cut out, as seen in FIGS. 5E and 5F.
The resolving power of an optical system is generally represented by the following equation:
LW=kxc2x7xcex/NA
wherein the reference symbol LW represents the line width at the resolution limit, k represents a constant, xcex represents the wavelength, and NA represents the numerical aperture of the projection optical system. As seen from the above equation, the line width at the resolution limit is inversely proportional to the numerical aperture of the projection optical system. It is necessary that the resolving power be constant regardless of the position in the wafer plane or the direction of the pattern (vertical, horizontal or inclined).
If the slit S is placed near the reflection mask or the work, vignetting is produced in a portion of the light flux at the exposure area near the edge of the slit S. Since resolving power varies according to the direction of the pattern, it is not possible to obtain an accurate image. In the scanning exposure method, because the scanning exposure is performed also in areas where an inaccurate image is produced, the difficulty is that the image in the entire illumination area becomes less accurate.
If a change in the magnitude or shape of the exposure area or illumination area is desired, it is necessary for the slit itself to be movable. In this case, it is impossible to project an accurate image because of problems such as those described below. In order to prevent interference between the movable mechanism of the slit S and the reflection mask or the work, the distance between the slit S and the reflection mask or the work increases. Also, dust is produced by the movable mechanism of the slit S.
Accordingly, it is an object of the present invention to provide an accurate image even in cases where a reflection mask is used.
In order to achieve the object described above, a projection exposure apparatus according to an embodiment of the present invention comprises:
a light source providing radiation light;
an illumination optical system directing said radiation light to a reflection mask on which
a predetermined pattern is formed;
a projection optical system directing said radiation light reflected by said reflection mask to an exposed substrate (work) for forming an image of a predetermined pattern on said exposed substrate; and
a scanning driver relatively moving at least one of said reflection mask and said exposed substrate;
wherein said illumination optical system includes a field stop located near a position substantially conjugate with said reflection mask.
In order to achieve the object described above, a projection exposure apparatus according to another embodiment of the present invention is used for directing radiation light to a reflection mask on which a predetermined pattern is formed, and for projecting an image of said predetermined pattern onto a work by said radiation light reflected by said reflection mask, the projection exposure apparatus comprising:
a radiation light source;
an illumination optical system located between said radiation light source and said reflection mask, and directing said radiation light from said radiation light source to said reflection mask;
a projection optical system located between said reflection mask and said work, directing said radiation light reflected by said reflection mask to an exposed substrate, and forming the image of said predetermined pattern on said exposed substrate;
a scanning driver relatively changing the positional relationship between at least one of said reflection mask and said work, and said projection optical system; and
a field stop located near a position conjugate with said reflection mask.
In order to achieve the object described above, an illumination apparatus according to another embodiment of the present invention is designed for use in a photolithography apparatus forming an image of a reflection mask on which a predetermined pattern is formed on a work, the illumination apparatus comprising:
a radiation light source; and
an illumination optical system located between said radiation light source and said reflection mask, to direct said radiation light provided from said radiation light source to a predetermined illumination area on said reflection mask;
wherein said illumination optical system includes a field stop and a relay optical system forming an image of said field stop on said reflection mask as said predetermined illumination area.
In the embodiments described above, it is desirable for said relay optical system to be a catoptric system. In any embodiment described above, it is desirable for said radiation source to provide radiation light having a wavelength shorter than 50 nm.
In order to achieve the object described above, a projection exposure apparatus according to another embodiment of the present invention is designed for directing radiation light to a reflection mask on which a predetermined pattern is formed, and for projecting an image of said predetermined pattern on a work by said radiation light reflected by said reflection mask, the projection exposure apparatus comprising:
a radiation light source;
an illumination optical system located between said radiation light source and said reflection mask, for directing said radiation light provided from said radiation light source to said reflection mask;
a projection optical system located between said reflection mask and said work, directing said radiation light reflected by said reflection mask to an exposed substrate, and forming the image of said predetermined pattern on said exposed substrate;
wherein a position of at least one of the reflection optical elements located along the optical path of said radiation light in said illumination optical system is adjustable in order to adjust illuminance distribution of said radiation light on said exposure area.
In order to achieve the above described object, a method according to another embodiment of the present invention is a method of projecting and exposing a predetermined pattern formed on a reflection mask onto a work comprising the steps of:
providing a radiation light source;
illuminating a reflection mask on which a predetermined pattern is formed with said radiation light source by means of an illumination optical system;
projecting said pattern of said reflection mask onto a work based on reflected light from said reflection mask; and
moving at least one of said reflection mask and said work relative to the other; wherein said illuminating step further comprises a step of forming an image of a field stop placed in said illumination optical system on said reflection mask.
In order to achieve the object described above, a projection exposure method according to another embodiment of the present invention is a method of projecting and exposing a predetermined pattern formed on a reflection mask onto a work comprising the steps of:
providing a radiation light source;
illuminating a reflection mask on which a predetermined pattern is formed with said radiation light by means of an illumination optical system;
projecting said pattern of said reflection mask onto a work based on reflected light from said reflection mask;
adjusting the position of at least one optical element located along the optical path where said radiation light is directed to said field stop in order to change the illuminance distribution in said exposure area on said work.
In the present invention, radiation light means electromagnetic waves including the deep ultraviolet, vacuum ultraviolet, soft x-ray, and hard x-ray areas.